1. Field of the Invention
The present invention relates to solenoid operated unit fuel injectors for internal combustion engines. In particular, to such fuel injectors which are capable of having distinct timing, metering and injection periods, and in which the same supply line serves for the delivery of fuel to both a timing chamber and a metering chamber.
2. Description of Related Art
Solenoid operated fuel injectors, of the type to which the present invention is directed, have been used for some time, and an example of such an injector can be found in commonly-owned U.S. Pat. No. 4,531,672 to Smith. In this type of injection, a timing chamber is defined between a pair of plungers that are reciprocably displaceable within the bore of the body of the injector and a metering chamber is formed in the bore below the lower of the two plungers. A supply rail in the engine delivers a low pressure supply of fuel to the injector body. To control this supply of fuel, a solenoid valve is disposed in the flow path between the fuel supply rail and the injector bore and the plungers block and unblock respective ports leading from injector body fuel supply circuit into the timing and metering chambers.
During the operation of such an injector, the port to the timing chamber is opened during retraction of the plungers to allow fuel to enter the timing chamber. During the injector downstroke, the timing port is closed by the upper plunger, and then, the metering port is opened to direct the supply of fuel into the metering chamber. During the entire time, from the start of the timing period through the end of the metering period, the solenoid valve remains open. As a result, during the portion of the downstroke before the timing port is closed, the downward plunger movement produces a high pressure backflow of fuel from the timing chamber, through the solenoid valve to the supply rail. Not only can this high pressure backflow damage O-ring seals within the injector, but it creates pulsations in the fuel supply rail that can result in a phenomenon known as "crosstalk", whereby the pressure wave produced by the backflowing injector causes "bumping" of an inlet check ball valve of other injectors of the engine so as to exert an influence on the quantity of fuel metered in the other injectors.
In an existing injector design, sold by the Cummins Engine Co. under the CELECT trademark, shown in FIGS. 1-3 & 3a, improved performance is achieved, and the timing fluid backflow-related problems ameliorated. In this existing fuel injector 1, as shown in FIG. 1, initially, during the retraction stroke, with the solenoid valve 3 closed, the metering plunger 5 and the timing plunger 7 rise together, and fuel under rail pressure is metered into the metering chamber 9. When the proper quantity of fuel has been metered, the solenoid valve 3 is opened, allowing fuel to flow into the timing chamber 11, causing the pressure at the top and bottom of the metering plunger to be equalized, thereby stopping movement of the metering plunger 5 while the timing plunger 7 continues to rise, and the timing chamber 11 to fill, as the retraction stroke is completed. During the downstroke, prior to the time at which injection is to commence, as shown in FIG. 3, the solenoid valve 3 remains open and fuel is forced back out of the timing chamber 11, through the solenoid valve 3 into the supply circuit.
However, unlike the situation in the injector of the Smith patent, a relief valve assembly 15 is provided to vent high pressure spikes from the rail side of the injector 1 to the drain side thereof (FIG. 3A). More specifically, the relief valve assembly 15 comprises a valve member 15a which is urged against a relief port 15b by a coil spring 15c which is disposed in spacer member 17, the upper surface of which forms the bottom wall of the metering chamber 9 and which contains channels through which fuel flows between the fuel inlet port 19 and the metering chamber 9 and from the relief valve 15 to a drain passage 21. When the pressure of the backflowing timing fluid exceeds that of spring 15c, the valve member 15a unblocks relief port 15b, thereby opening a path from the fuel supply circuit to drain passage 21.
Similarly, at the end of the injection phase, when the solenoid 3 is closed, the top edge of the metering plunger 5 passes below at least one timing fluid spill port 23, thereby evacuating the timing chamber 11 via the drain passage 21. Additionally, passages 5a in the metering plunger 5 are brought into communication with at least one spill port 25 by which a small quantity of fuel is spilled to the fuel supply circuit. To prevent pressure spikes due to the fuel spilling from the metering chamber 5, valve member 15a, again, is forced open to vent the excess fuel pressure from the supply side thereof to the drain passage 21.
On the other hand, while a definite improvement over other prior art injectors, it has been found that the valve assembly 15 does not fully resolve the problems associated with pressure build-ups in the fuel supply circuit, and shot-to-shot fuel volumes can vary by as much as a third during engine idling conditions, thereby adversely impacting on idling emissions from the engine with which the fuel injector is used. These inconsistencies appear to be due to the length and tortuous nature of the path of the supply side fuel routing to the valve assembly, which affects the time it takes for the pressure wave to reach valve member 15a and the pressure of the fuel when it does reach valve member 15a. However, the space requirements for such a spring-loaded valve assembly 15 and the limited space available for it to be incorporated into the injector, prevent the problems associated with the use of valve assembly 15 from being fully addressed by merely shifting its position to shorten and simplify the flow routing to it. Furthermore, the use of valve assembly 15 is associated with the costs of the high degree of precision machining required to produce it and the flow paths to and from it, as well as that attributable to production and assembly of the three parts thereof (i.e., valve member 15a, valve spring 15c and the threaded plug 15d used to hold them in place).
One-way, spring valves, which permit a fluid to flow therethrough in only a given direction and as a function of the extent to which the pressure of the fluid acting in prescribed flow direction exceeds the force of the spring in a valve closing direction, have also been known for a long time. Such valves in which a band-shape spring serves as the valve spring have been used in numerous types of equipment, from air compressors (U.S. Pat. No. 233,432) to controls for load-moving mechanisms (U.S. Pat. No. 4,095,617). In various different types of fuel injectors such spring valves have been used to control the supply of fuel to a fuel injector nozzle (U.S. Pat. Nos. 2,590,575 and 5,014,918) as well as the releasing of timing fluid from a timing chamber (commonly assigned, co-pending U.S. Pat. application Ser. No. 07/898,818 to Kolarik et. al.). However, because of the construction of the noted CELECT injectors, such a band spring valve cannot merely be substituted for its relief valve assembly 15; furthermore, more than mere adaption of the barrel member for the use of such a valve would be required to overcome the noted shortcomings of the valve assembly 15.
Thus, there still is a need for further improvements to fuel injectors of the type to which this invention is directed, both from the standpoint of reducing supply side pressure effects, and from the standpoint of simplifying the construction and costs of producing such fuel injectors.